Waveguide-type optical amplifiers doped with Ytterbium and Erbium ions are theoretically studied. Sensitization of Er-doped amplifiers with Yb ion doping have many advantages such as the possibility of using broader pumping wavelength range and efficient pumping with smaller pumping power. Transient amplification characteristics of optical short pulses are numerically analyzed using FDTD method. The amplification characteristics are compared with the result of the steady state analysis using the rate equations.